The medical relief teams racing to help victims of the earthquake in Haiti are up against a grim biological fact: People trapped and injured are most likely to survive if rescued within 48 hours, and very few people are found alive more than six days after a disaster.

The rare people who are rescued much later often need treatment -- kidney dialysis, intensive-care nursing and cardiovascular support -- that is singularly hard to deliver in disaster zones.

Yet, the landscape of many recent natural disasters has featured not only wreckage and bodies, but also the high-tech encampments of surgical teams that arrived too late to be of much use while many more ordinary medical needs continued to go unaddressed.

"The real issue is you have to know what is going to be needed at the time you can realistically arrive and be ready to operate your medical resource," said Joseph A. Barbera, an emergency physician and co-director of George Washington University's Institute for Crisis, Disaster and Risk Management.

He left Thursday as part of a team from the Fairfax County urban search and rescue squad, one of the few relief organizations with experience treating victims of "crush syndrome" in disaster zones. People suffering from that problem can die within minutes of being rescued unless specific treatment -- often begun before the debris is lifted -- is performed.

The flood of uncoordinated "medical assets" was particularly dramatic after the Indian Ocean tsunami in December 2004. It was one reason the World Health Organization offers unusually blunt advice to medical do-gooders.

The Geneva-based organization's Web site lists under "inappropriate response" to earthquakes:

"Medical or paramedical personnel or teams: Do not send them! They would arrive too late. Local and neighbouring health services are best placed to handle emergency medical care to disaster victims. Field hospitals, modular medical units: Do not send them!"

Whether that advice is appropriate at the moment for Haiti -- a country with a poor medical system (much of it now damaged) and a population with high rates of illness and poor nutrition -- is unclear. It is not stopping relief organizations and charities from sending physicians, nurses, logistics experts, publicists and supplies.

A spokesman for the Health Action in Crises section at WHO said Thursday that no organization has been explicitly told not to go to Haiti. He said, however, that there is far more coordination now than in earlier disasters.

"We are not actively discouraging, but we are advising and we're not seeing uncoordinated, dysfunctional and over-the-top provision of supplies and care," Paul Garwood said. "Since the tsunami, there has been a huge reform of humanitarian work in emergencies."

The coordination is being done by a "health cluster" whose 80 members include U.N. agencies, large nongovernmental organizations and charities with health expertise. A WHO physician is scheduled to arrive in Port-au-Prince on Friday to lead the cluster, Garwood said.

Some experts who have worked in the country predict that even experienced relief groups will face severe problems making themselves useful.

Dartmouth College President Jim Yong Kim, a physician and co-founder of Partners in Health, an organization that has worked in Haiti for years, said few groups know how to operate there.

"You can't go into Haiti with a plan that is 85 percent or 90 percent realized and expect to get a little bit of help on the ground," he said. "There is no infrastructure on the ground to help. So if you send in a plane fully loaded but expect someone on the ground to help unload it, you are going to find it is impossible."

In a 2006 study of 34 earthquakes going back to the early 1980s, researchers at GWU found no credible reports of people who survived 14 days after the disaster. There were five live rescues at least 10 days after collapse, and 42 between five and 10 days after. Fatalities ranged from fewer than 100 in six earthquakes to five with more than 15,000. The largest temblor in the study occurred in Iran on June 20, 1990, and killed 40,000 to 50,000 people.

Barbera, one of the GWU researchers, is hopeful there will be numerous late "saves" in Haiti.

"With devastation this wide, and the number of structures affected, one would expect there would be rescues for at least another three to five days. It's possible," he said.

Rescuers speak of a "golden 48 hours" for saving earthquake victims, analogous to the "golden hour" for starting treatment in people with penetrating trauma and shock. That time, however, has passed in Haiti.